This invention relates to an extruder screw for use in the barrel of a plastics material extruder and is more particularly directed to the construction and design of the portion of the screw beginning with the initial fill with unmelted plastic material, through the initial melting and plastic material compacting stage and through and including the first dispersive mixing stage in which the molten plastic material traverses a melt barrier flight and runs into and along a channel separate from the channel for compacted unmelted plastic material.
The concept of providing separate flighting and resulting separate channels for melted and unmelted plastic material, in the initial stages of an extruder screw is shown a number of prior patents, including Chung et al, U.S. Pat. No. 4,405,239, issued Sep. 20, 1983; Hsu, U.S. Pat. No. 3,858,856, issued Jan. 7, 1975; Chung, U.S. Pat. No. 4,000,884, issued Jan. 4, 1977; and Barr, U.S. Pat. No. 3,698,541, issued Oct. 17, 1972.
A particular difficulty of many prior designs is the fact that in the development of the flighting to create the solids compaction channel separate from the melt channel causes a sudden change in the pitch or the depth of a developing channel and results in a flow disruption, a discontinuity, and often the packing of the melt channel with solids. These disruptions can cause a variation in pressure, consistency and also, rate of output of the screw.
While prior designs have used a barrier flight to separate a melt channel from a solids compaction channel, the barrier flight has commonly had a constant clearance with the barrel, with the result that a much higher specific energy input is applied at the beginning of the barrier section when the melt viscosity is high than at the end of the melt channel when the viscosity of the melted phase is much lower. Such an arrangement inherently causes a non-uniform shear energy input to the melt over the length of the barrier flight. As a result, that polymer initially traversing the melt barrier experiences a high shear energy input and a long residence time at the elevated melt temperature while that polymer passing over the melt barrier at the end of the solids channel experiences a lower shear energy input, and a reduced barrier at the end of the solids channel experiences a lower shear energy input and a reduced residence time at elevated melt temperatures creating an inconsistent melt quality. The present invention eliminates this inconsistency.
Further, in the transition between a barrier section and a conventional metering section, designs have been proposed in which there has been no thought of maintaining a balance of pressures between the compaction channel carrying the solids and the adjacent melt channel, nor has provision been made to accommodate polymers of varying melt rates. An abrupt transition at the end of the solids channel can cause a pressure fluctuation, adversely affecting the performance of the screw.